There is a public commuter bus service between Denver and Fort Collins: Bustang North Line. It runs from Denver Union Station (which is where Amtrak stops) to the downtown Fort Collins transit hub, with just two intermediate stops (I-25 on US-34 in Loveland, and the Harmony Transfer Center on I-25 and Harmony Road in Fort Collins).
The bus operates mainly during travel hours, with only a few buses on weekends. As of this writing, the last northbound service leaves Union Station at 7:00 p.m. Monday through Friday or 7:30 p.m. Sundays. Since Amtrak long distance trains often arrive late, you would want to have a backup plan, but this could be a convenient and inexpensive option if you are on time.
The fare from Denver to Fort Collins is $ 10. The bus has a large trunk underneath and free WiFi.